The owner of British Airways has signed a letter of intent to order 200 Boeing 737 MAX jets - the aircraft model currently grounded following two deadly crashes.
International Airlines Group (IAG) - which also owns Spain's Iberia and Ireland's Aer Lingus - said it had every confidence in Boeing and expected the MAX to return to service in coming months.
The deal - worth more than $24bn (£19.1bn) at list prices though likely to have been reduced in negotiations between the two companies - would see the aircraft delivered between 2023 and 2027.
It was announced at the Paris airshow.
Boeing's entire 737 MAX fleet was grounded in March following two crashes in which a total of 346 people died.
The US company has been working on a software fix to try to return the jet to service by the end of the year and said the decision of when it flies again was in the hands of regulators.
IAG said its deal with Boeing - which remains subject to formal agreement - would see the aircraft used by a number of its airlines including Vueling and Level plus BA at Gatwick.
Chief executive Willie Walsh said: "We have every confidence in Boeing and expect that the aircraft will make a successful return to service in the coming months having received approval from regulators."
Mr Walsh, a former 737 pilot, said he would have "no hesitation" in flying on the plane.
Fixes to the model relate to its MCAS anti-stall software, which investigators have put at the heart of the problems which caused the two crashes.
Mr Walsh said he had seen the software in person, adding that it was "very helpful to see it in operation" and to "understand the changes".
IAG is one of the world's largest airline groups, carrying 113 million passengers in 2018.
Kevin McAllister, Boeing's head of commercial aircraft, said in response to the deal: "We can't thank you enough for the confidence you placed in the Boeing family."
Shares in Boeing closed more than 5% higher following the announcement. IAG was up 1.5%.